Page-based volume holographic storage has emerged as a promising new data storage approach. For information on conventional holographic storage see for example U.S. Pat. Nos. 4,920,220, 5,450,218, 5,440,669, and 5,377,176. In holographic storage, data is stored as a hologram resulting from the interference of a signal and reference beam. During storage, both the reference and signal beams are incident on the storage medium. During retrieval, only the reference beam is incident on the medium; the reference beam interacts with the stored hologram, generating a reconstructed signal beam proportional to the original signal beam used to store the hologram.
Each bit is typically stored as a hologram extending over the entire volume of the storage medium. Multiple bits are encoded and decoded together in pages, or two-dimensional arrays of bits. Multiple pages are stored within the volume by angular, wavelength, phase-code, or related multiplexing techniques. Each page can be independently retrieved using its corresponding reference beam. The parallel nature of the storage approach allows high transfer rates and short access times, since as many as 10.sup.6 bits within one page can be stored and retrieved simultaneously. Spatial multiplexing can be used to increase capacity through the use of multiple, independent storage subvolumes.
A data encoder situated in the signal beam path is normally used to encode data pages in the signal beam. The data encoder can be an amplitude spatial light modulator (ASLM) having a 2-D cartesian array of square regions corresponding to the data to be stored. The transmissivity of each region can be individually and dynamically adjusted. The data encoder can be situated in a Fourier or near-Fourier arrangement relative to the medium.
Disadvantages of conventional holographic storage systems include potential sub-optimal stacking of storage subvolumes in systems using spatial multiplexing, as well as non-uniform spatial frequency filtering of the signal beam. Such spatial frequency filtering may lead to loss of information encoded in the high-spatial-frequency components of the signal beam.